The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of fluid, such as a pump, to a fluid pressure operated device, such as a vehicle steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter and valving and an arrangement for imparting follow-up movement to the valving in response to flow through the fluid meter. The flow through the controller valving is directly proportional to the area of the main variable flow control orifice which, in turn, is proportional to the rate at which the steering wheel is rotated.
The assignee of the present invention now commercially sells fluid controllers, under the trademark Q-AMP.RTM., in which a known, predictable proportion of the inlet flow passes to the steering cylinder without flowing through the fluid meter, thus "amplifying" the flow in the sense that the total flow to the steering cylinder is greater than the flow capacity of the meter. Such controllers are known from U.S. Pat. No. 4,759,182, assigned to the assignee of the present invention and incorporated herein by reference. It is now known to be able to use such flow amplification controllers in applications where manual steering capability is required. In other words, the operator rotates the steering wheel and causes the fluid meter to operate as a hand pump, generating a flow of pressurized fluid to the steering cylinder, when there is no pressurized fluid present at the inlet of the controller. One way of facilitating manual steering with a controller of the flow amplification type is to close off the flow amplification path before the valving reaches its maximum displacement, which is the condition of the valving during manual steering. Unfortunately, the result is that during normal, powered steering, the flow amplification feature is gradually lost as the valving approaches its maximum displacement.
Another approach to achieving manual steering in a flow amplification controller is to place a check valve in the flow amplification path, such that it is able to prevent reverse flow of fluid in the amplification path, which otherwise would effectively "short circuit", back to the inlet port, the pressurized fluid being generated by manual rotation of the steering wheel. The use of such a check valve to facilitate manual steering is illustrated and described in U.S. Pat. No. 4,838,314, now assigned to the assignee of the present invention, and incorporated herein by reference.
In the typical fluid controller of the type described, the valving comprises a rotatable primary valve member (spool) and a relatively rotatable follow-up valve member (sleeve). The assignee of the present invention now also has commercially available a fluid controller in which the rotational displacement between the spool and sleeve can in the range of about 40 degrees to about 50 degrees, rather than the 10 degrees or 20 degrees which was typical in prior art controllers, and in some cases the displacement can be as much as 60 degrees. Such controllers are referred to as "wide angle" controllers, and a primary benefit of a wide angle controller is the ability to substantially reduce the "lateral jerk" which would previously occur when the vehicle operator had been steering in one direction, then returned the steering wheel to its centered position, thus closing the controller valving. Such wide angle controllers are illustrated and described in U.S. Pat. No. 5,080,135, also assigned to the assignee of the present invention and incorporated herein by reference.
One of the problems associated with fluid controllers has been the inability to provide a unit having wide angle architecture, but also having flow amplification capability, and with the ability to manually steer without having to close off the flow amplification path as the valving approaches maximum displacement. Prior to the present invention, it appeared that it would be necessary to choose among wide angle capability, flow amplification, manual steering capability, and keeping flow amplification open at maximum valve displacement, but that it would not be possible to have all of the above in a single unit.
In addition, certain flow amplification controller designs encountered the phenomenon of flow amplification "feed-through", in which, under certain operating conditions, fluid is fed through the flow amplification path to the steering cylinder, imparting steering movement to the vehicle, even though the steering wheel is not then being rotated.